P.S The .xhtml file extension is configured and associated property in the Content Types Preference Page.

Solution

Thousand time of tries and errors, but still unable to get rid of the “unsupported content type” error, may be it is caused by the Eclipse IDE itself ? Give up, just upgrade to the latest Eclipse Helios (3.6), solve the problem immediately :)

Note
Not sure what’s the root cause of it, may be i had installed too many plugins, and some caused the conflicts. Just get a clean copy of the Eclipse IDE will get rid of the annoying error message immediately.